1. The Field of the Invention
This invention relates to valves used to control the flow of fluids. More particularly, the present invention relates to valves used to control the flow of erosive or corrosive fluids.
2. The Background Art
Valves of various types are crucial components in many industrial process and fluid transportation systems. In many instances, a valve will be required to handle an erosive, corrosive, or other fluid or slurry which is incompatible with metallic components. In order to handle such fluids, it is often necessary to line pipes and other components which come in contact with the fluid with materials such as plastic or ceramic materials.
Valves which come into contact with erosive or corrosive fluids present particular problems. Since the components of the valve must move to control the fluid flow, it is a particular challenge to make a valve which can carry out its fluid flow control functions while being fabricated from a material which is compatible with the fluid.
In particular, valves which control the flow of erosive or corrosive fluids require that liners, seats, plugs, balls, and other wetted parts, be made of compatible, i.e., non-metallic, materials. The problems which have hindered the use of non-metallic wetted parts often arise because of the weakness of the non-metallic components in the valve. In particular, the interface between the metallic actuator components and the non-metallic flow controlling element often presents problems.
For example, a rotary ball valve includes a rotating spherical ball, provided with a cylindrical passageway therethrough, which acts as a flow control element in cooperation with a conical seat formed in the valve body. A shaft or stem connects the spherical ball element, through appropriate seals, to an external actuator. The stem transmits the force necessary to turn the spherical ball to control the flow of fluid through the valve.
FIG. 1 is a diagrammatic representation of one such rotary ball valve as is known in the art. The valve represented in FIG. 1 includes valve housing components 14 and shaft 20, The rotation of the shaft 20 controls the flow of fluid through the valve. The valve housing components 14 and the shaft 20 can be fabricated from steel or other material known in the art. The valve housing components 14 and the shaft 20 do not come into regular contact with the fluid and thus can be fabricated from conventional materials.
A seating assembly 12 is fabricated from a material which is compatible with the fluid, for example, a ceramic material. A spherical ball element 10 is positioned within the seating assembly. The spherical ball element 10 is also fabricated from a material which is compatible with the fluid, for example, a ceramic material. The spherical ball element 10 is provided with a passageway, shown by the dashed line at 16, through which the fluid passes when the valve is in the open position as illustrated in FIG. 1.
When fluid is impinging upon the valve in the direction of arrow F, dynamic forces caused by the flow of the fluid through the valve, and static forces developed when the valve is shut off, as well as the differential pressures which are generated, forces the spherical ball element 10 against the seating assembly 12. The spherical ball element 10 and the seating assembly 12 together form a sealing relationship at the location represented at 18 (which has been represented in a slightly exploded configuration to show the pertinent structures).
In addition to the forces just described, frictional forces are created by the contact of the spherical ball element 10 and the seating assembly 12. Additional forces are also created from contact of the spherical ball element 10 with residual material deposited from the fluid in contact with the spherical ball element 10. In particular, high frictional forces can develop when an abrasive material is included in the fluid, e.g., a slurry, coming in contact with the valve. The abrasive material can become trapped between the sealing interface 18 between the spherical ball element 10 and the seating assembly 12 in sufficient quantity to encapsulate the spherical ball element 10. All of these described conditions require that additional torque be applied to rotate the spherical ball element 10.
When non-metallic materials are used for the spherical ball element 10, for example a ceramic material, high compression strength to provide erosion resistance is often provided, but, disadvantageously, such materials often exhibit low toughness and tensile strength. Most disadvantageously, in the previously available devices, the interface between the steel shaft 20 and the spherical ball element 10 is prone to fracture and failure during application of the high torque needed to overcome the conditions described earlier and operate the valve.
As illustrated in FIG. 1, a key 22 formed on the end of the shaft 20 which is received into a corresponding key way formed on the central axis of the spherical ball element 10 is the general structure used in the prior devices to interface the two components. Alternatively, some prior devices utilize a hexagonal shaped key. The torque applied to turn the shaft 20 and the spherical ball element 10 during operation of valve can cause the fracture of the spherical ball element 10 resulting in failure of the valve.
In view of the forgoing, it would be an advance in the art to provide a more reliable valve utilizing a non-metallic spherical ball element which is less prone to fracture and breakage.